Just jumping into this story in progress to say a few things. First, if sound is your priority then I recommend building the room from the subwoofer up. The reason is that the sub region is strongly influenced by placement of the subwoofer and your listening position. If you determine sub and listening position first then build everything around it, you'll have a sonically stable foundation. Above the crossover frequency is easier to manage with absorption, worst comes to worst, so you have more flexibility there.
38% is a depreciated standard. In fact, I never found it perfect. Credit where credit is due, Anthony Grimani made a strong case for 20, 32 and 45% each axial dimension and I find that much better and more flexible. That's for your listening position. Mark those off with tape and put your sub in a primary trihedral corner like your front-side wall. Take measurements and juggle the positions until you find the maximum gain at the lowest frequency with a minimum of nulls below the crossover point (~85Hz). Your sub will probably be somewhere along your front wall but not centered and not too far away. About a 1/4 wavelength is not unreasonable; it's also a valid location despite what some might say. After you've gotten the best response, you can add your mains, absorption, and everything else. Measure again, tweak and integrate: done.
What I can tell you about absorption is this: you can predict room modes and I suggest you find a calculator for doing so. Each dimension supports a fundamental resonance of double its distance and the resonances follow the harmonic series from there. Your first four modal harmonics are the biggest dogs so just focus on those. Now here's a key point: you don't necessarily need to fully absorb all your ringing. In other words, some people say you need 1/4 wavelength-thick frictional absorption to treat your bass. Not so! It depends on your targets and needs and the absorptive/transmissive properties of your boundaries. If you plan on adding a lot of absorption because it's a multi-purpose room, like a home studio, then sufficient coverage of thinner material can get you to where you want to be. My target is 20dB decay within 150ms above 63Hz with a smooth taper, longer toward the bottom. The easiest way to do this is about 15% coverage with sufficiently thick absorption. The three classes of frictional absorbers are rigid like OC703, semi-rigid like Safe n' Sound, and fluffy like pink R19. Thickness per material type matters and the ideal thicknesses for each material are 6", 12" and 24" respectively. You can gap the material 1x thickness from the wall for lower octave performance with a slight penalty in the lower mids. Additionally, you can add FRK to non-early reflection points to further enhance the performance via a damped pressure element. If you are tight on space, either use a lot of 6" rigid or invest in pressure absorbers if needed. I'm of the school that more is better and have seen that very few, if any, people proceed with absorption to the point where their room sounds "too dead" as some claim. A huge mistake is covering your whole place with 2" foam - don't do that! Thicker is better as long as you don't exceed the thickness-type limits I set or you will defeat the point and introduce reflectiveness due to excessive resistance.
My contribution here is primarily positioning and absorption. I've yet to seriously apply EQ to my setups. That said, your biggest and cheapest tool is positioning. Spend time getting that right and everything else will be easier. Bear in mind that you want symmetry from your ears-forward to get the best stereo imaging. This contradicts with a 45% listening position left-right. That's ok - balance it out. Room acoustics are exactly a balancing act the whole way. It's not that hard - just making an effort will get you the majority of the result.
Lastly, SBIR in practice is mainly about nulls. Specifically 1/4 wavelength distances from boundaries. In a bass-managed system (with sub), you want your mains crossed over around 85Hz and their placement either close to (5cm) or far from a boundary (>1.1m). The idea is to get the null lower than the crossover. SBIR also creates peaks but they're less problematic. See
Genelec's materials for more. Basically, don't have your monitors equidistant to more than one boundary or the null will compound and you'll have a harder time getting rid of it.
Oh, how could I forget - room gain! Yeah, having any speaker or sub close to a boundary increases the bass. That's no problem because you can offset it with EQ. You can however use it to your advantage to improve low end performance where it might otherwise be lacking. Free bass is good (not freebase though, that's not good).
Let me add that for frequencies below about 300Hz, you don't want to smooth your bass response in REW. Unsmoothed lets you have the raw, honest detail so you can make fine tweaks. Smoothing is ok above the lowest octaves. I aim for +-10dB or better. Add a house curve if you wish. Use EQ to cut but boost sparingly.
